Synthetic polymer that is made from petroleum as raw material and has been produced with very large amount on the earth causes social problem for disposal of waste polymer, because it emits poisonous gas when it burns and does not degrade in natural surroundings. Additionally, it is pointed out that petroleum-made plastics, such as polystyrene or polycarbonate, contain an endocrine disrupter which may injure living of the human beings. Other plastics often contain some oligomer, but it has been warned that the oligomer may harm the human bodies.
It is also noted to replace petroleum plastics with agricultural product, especially starch material, in view of energy and resource strategy after exhaustion of petroleum resource and zero-emission system of carbon dioxide.
Thus, instead of the synthetic polymeric material obtained from petroleum resources, polymeric materials that are obtained from starch or wood has been developed, because they are harmless to human bodies and do not destroy nature. These products have been used for years and are safe to human bodies. They are also buried in soil and degraded with bacteria or microorganism.
Some products formed from starch have already been available, such as a cushioning which is produced by the steps of extrusion-forming starch in the presence of water, and such as trays or cups which are produced by heat-forming and molding of starch slurry. The starch products are, however, poor in water resistance and strength characteristics, in comparison with those obtained from synthetic polymers. It is also proposed that starch is mixed with other biodegradable synthetic polymers to form into film, sheet or molded article, but the final products do not have sufficient properties as required for industrial products. It is still desired to develop starch products having sufficient physical and chemical properties equal to those obtained from petroleum based synthetic polymers.
There are some problems inherent to starch products as follow:
(a) Natural starch is generally composed of a mixture of amylose (polymer having a construction of linearly bonded glucoses) and amylopectin (bunch-shape polymer composed of amylose having branches). The linear amylose has good processability, film properties and molding abilities equal to synthetic plastics, but the amylopectin shows poor strength characteristics. Thus, natural starch which is a mixture of amylose and amylopectin has poor strength properties.
(b) Amylose content of natural starch is not so high. For example, corn starch has amylose content of as low as about 25% and even high-amylose corn starch has an amylose content of about 70% or less. Thus, such natural starch provides a poor molded article.
(c) Amylose can be extracted or separated from natural starch, but its process is complicated and yield of amylose is very low. The process does not have industrial cost effectiveness.
(d) Amylose present in natural starch generally has a molecular weight of as low as about ten thousand Da to hundred thousand Da. For example, amylose in corn starch has a molecular weight of 250000 Da (250 kDa), and amylose of starch in Irish potato has a molecular weight of 490000 Da (490 kDa). It is known to the art that amylose with low-molecular weight is easily retrogradated and shows poor mechanical strength. Accordingly, ever if amylose is effectively extracted or separated from natural starch, the resultant amylose dose not has sufficient characteristics for substituting plastics.
(e) Amylose present in natural starch has a broad molecular weight distribution (Mw/Mn) of greater than or equal to 1.3. A molded article which made by amylose having a broad molecular weight distribution has poor strength properties and processability.
(f) Amylose present in natural starch does not have complete linear structure, but has small amount of branched structure. Thus, the nucleation speed of natural amylose is fast and the natural amylose easily crystallizes by itself. The characteristics of natural amylose make the structure of film or sheet nonuniform and significantly reduce transparency and mechanical strength.
(g) Amylose present in natural starch easily dissolves in hot water of greater than or equal to 130° C., but precipitates at a temperature of less than 130° C. (re-crystallization) to form cloudy solution because of the reasons mentioned in the above (d), (e) and (f). The molded article obtained therefrom also has a nonuniform structure and shows poor processability, poor transparency and poor strength.
(h) Amylose present in natural starch does not easily dissolve in water at ambient temperature, but dissolves in specific organic solvent, such as dimethylsulfoxide and dimethylformamide. The use of natural amylose requires the process to recover the organic solvent, so that the process is not good for manufacturing because of cost effectiveness. Absence of good and useful solvent is also a serious disadvantage of natural amylose upon altering its polymeric properties by chemical modification.
(i) In order to modify the polymeric characteristics of natural starch, it is also proposed to graft-polymerize starch molecules with vinyl monomer, such as methyl acrylate, methyl methacrylate, or styrene. The modification raises cost of production, but does not enhance polymeric characteristics so much. Additionally the vinyl graft portions do not show biodegradation.
(j) It is difficult for natural amylose to control swelling by chemical crosslinking reaction.
Industrial application of natural amylose does not proceed so much because of the above mentioned reasons.
In recent years, an approach for using synthesized amylose in place of natural amylose has been studied. One approach for producing synthesized amylose is bonding glucose by an enzyme to synthesized amylose (called enzyme synthesizing method).
In an example of the approach, it is proposed that sucrose is used as substrate and is treated with amylosucrase (EC 2.4.1.4), which is called AMSU method. In AMSU method, however, the resulting α-1,4-glucan has low-degree of polymerization. It has been reported that even if highly purified amylosucrase is employed, the resulting amylose has a molecular weight of 8,941 Da (see FEBS Ltters 471, Montalk et al, pp 219 to 223 (2000)).
In AMSU method, the resulting α-1,4-glucan has low average molecular weight such as several thousand Da, even if the α-1,4-glucan has narrow molecular weight distribution. α-1,4-glucan having a molecular weight of less than several tens of thousand easily precipitates, and it is hard to form a molded article to employ the α-1,4-glucan. On the condition that a molded article can be formed by employing the α-1,4-glucan, there is a problem that the resulting molded article does not have sufficient strength properties.
As an enzyme-synthesizing method other than the AMSU method, a method using glucan phosphorylase (α-glucan phosphorylase, EC 2.4.1.1.: generally called phosphorylase) is proposed. The enzyme-synthesizing method includes a method reacting phosphorylase with a substrate (glucose-1-phosphate: G-P-1) and transferring glucosyl unit from glucose-1-phosphate to a primer (malto-heptaose), which is called as GP method. The enzyme-synthesizing method also includes a method reacting sucrose phosphorylase in addition to phosphorylase with sucrose to synthesis G-P-1, and transferring glucose of G-P-1 to a primer, which is called as SP-GP method. (see, for example, WO 02/097107 pamphlet.)
WO 02/06507 pamphlet discloses an article formed from enzyme-synthesized amylose with a weight average molecular weight of greater than or equal to 100 kDa and a molecular weight distribution (Mw/Mn) of not more than 1.25. The enzyme-synthesized amylose includes no amylopectin which deteriorates strength properties of the resulting molded article. In addition, the enzyme-synthesized amylose is composed of completely-linear amylose, which can not be accomplished in natural amylose. Furthermore, the enzyme-synthesized amylose can be designed for having a molecular weight distribution of not greater than 1.25, which is narrow molecular weight distribution and can not be accomplished in natural amylose. Thus, the method provides a molded article having excellent transparency, processability and strength properties. In the method, however, has a problem of difficulty of molding resulting from relatively-high water solubility and long gelation time of the enzyme-synthesized amylose having an average molecular weight of greater than or equal to 100 kDa.
WO 99/02600 pamphlet discloses a thermoplastic mixture. Claims of WO 99/02600 disclose that the thermoplastic mixture is obtainable by preparing and mixing;
(A) 100 parts by weight of a biocatalytically produced 1,4-α-polyglucan,
(B) up to 400 parts by weight of a melt-processable polymeric material different from (A),
(C) water in an amount sufficient for plastification of the mixture,
(D) at least one plasticizer in an amount of 10 parts by weight up to half the total of the parts by weight of (A) and (B), and
(E) optionally up to ((A)+(B)) parts by weight of other conventional additives, provided that the water content of components (A) and (B) has been corrected to zero by calculation.
The reason why component (A): 1,4-α-polyglucan is used for the preparation of the thermoplastic mixture in WO 99/02600 is not clear. In WO 99/02600, component (B) includes various materials such as proteins, starches, various polysaccharides and synthesis resins. Therefore, the invention described in WO 99/02600 is disparate from the present invention, that is two kinds of α-1,4-glucans (high molecular weight α-1,4-glucan and α-1,4-glucan having low-molecular weight) are mixed and thus provides a molded particle. Furthermore, to make a molded article using the thermoplastic mixture in WO 99/02600 requires a heat process. For example, making a molded article in an example described in WO 99/02600 includes heating process within the range of 100-160° C.